Small handheld electronic devices have become ubiquitous in society, and people are becoming more and more dependent on these devices to obtain information, communicate with others, provide entertainment, and perform financial transactions, among other things. The devices' small size makes them easy to lose and a target for thieves, while their increasing capabilities provide access to sensitive personal data and financial resources. Various security techniques have been developed to determine if a person trying to use such a device has been previously authorized to use the device. Biometric authentication is sometime proposed as a way to avoid passwords or artifacts (e.g., keys, badges, etc.), both of which can be stolen. But biometric identification can also be spoofed, by recording the authentic characteristic (a photograph of the face, an audio recording of the voice, a copy of a fingerprint, etc.) and using that recorded data to fool the biometric sensor.